IPv6 ULAs have a global scope, so when it comes to a default address selection in IPv6, longest prefix match criteria is used to chose a proper source IPv6 address to speak with the remote site within the same (global) scope.

I’ve discussed scopes and zones in one of my recent posts. I’ve mentioned IPv6 ULA in that context which was somehow misleading and well spotted in a comment by Roger Wilco, saying “Strictly speaking, ULA have global scope, and so the scope and zone math shouldn’t be required to be able to explain why an IPv6 host can want to speak with a remote globally routed address from a local ULA.” Absolutely true! Read the rest of this entry »

IPv6 is well designed. The model of scopes and zones along with the zone isolation principle is based on solid mathematical standards and can provide straight answers to tricky questions regarding packets with mixed source and destination address scopes. Can a packet with a link-local or ULA address reach the global destination? There is no doubt about that, at least not in IPv6 theory.

Ivan Pepelnjak was discussing the usage of ULA (Unique Local Addresses) recently in one of his blog post at ipSpace. He says: “If the destination IPv6 address is a global IPv6 address and the source host has an ULA address but no global IPv6 address, it tries to use the ULA source IPv6 address (and might reach the destination or not).”. To understand why this can actually work, it is necessary to have some insight about scopes and zone in IPv6, and the basic rules that dictate the packet forwarding within the scope zone. Read the rest of this entry »